Relaxation methods, multicomponent

The Kirkendall effect in metals shows that during interdiffusion, the relaxation time for local defect equilibration is often sufficiently short (compared to the characteristic time of macroscopic component transport) to justify the assumption of local point defect equilibrium. In those cases, the (isothermal, isobaric) transport coefficients (e.g., Dh bj) are functions only of composition. Those quantitative methods introduced in Section 4.3.3 which have been worked out for multicomponent diffusion can then be applied. 

Some applications are listed to illustrate these potentials. They may be classified in various ways. The most direct approach consists in working with labelled (deuterated), model systems. Real materials (designed for industrial applications and processed at a large scale) are often multicomponent, complex systems, which may be relatively ill defined at the molecular scale. Thus, working with chemically well defined, labelled materials (which, however, often have relatively poor mechanical properties by themselves) is a way to isolate and study the various parameters which play a role in rubber properties. Studies are done both in the relaxed state and in constrained (uniaxially deformed) states. This approach is illustrated in Section 15.3. Examples of studies performed in model, single component networks are presented. However, even in this case, the sensitivity of the method is such that it may detect the presence of a few percent of molecular defects. 

Porous silicas are usually mesoporous materials and they can be made with a variety of pore dimensions. In particular, silica glasses can be made with well-defined pore diameters, typically in the range 30-250 A, using sol-gel methods. Such a system provides a good model for testing the models of relaxation behaviour of fluids in porous solids. It is normally found that the two-site fast-exchange model for relaxation described above for macroporous systems is still valid. For instance, H and relaxation times have been measured during both adsorption and desorption of water in a porous silica. Despite hysteresis in the observed adsorption isotherms, it was found that the relaxation times depended solely on water content.For deuterated water in some porous silicas, multicomponent relaxation behaviour for T2 and Tip has been observed, and this has been attributed to the fractal nature of the pore structure. 

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